Both my merge splitting add-ins
Individual merge letters. and
Merge and Split will merge to PDF and allow you to set a common password (I am working on a version of the former that will allow you to set individual passwords from the merge data source, but it will not be ready for a while).
The process requires the addition of (free) third party software
PDFCreator 2.5.2 which is also linked from the web page. Anecdotal evidence suggests that it will also work with the more recent and more bloated version of the
PDFCreator software, but I haven't tested it and recommend instead the version 2.5.2.